a) Field of the invention
The present invention relates to an optical system which comprises graded refractive index lens element having refractive index distribution in the direction perpendicular to the optical axis.
b) Description of the prior art
Since light weight, and compactness are now demanded in combination with higher specifications for optical systems along with compact designs of electrical systems and mechanical systems used in various types of optical instruments, it is becoming difficult to design optical systems satisfying such demands only with homogenous spherical lens elements. Though there are known optical systems which use aspherical lens elements, no optical systems have not succeeded yet in overcoming the above-mentioned difficulty.
Under the circumstances described above, attention has recently been paid to optical systems in which graded refractive index lens elements are adopted.
Graded refractive index lens elements (GRIN lens elements) are classified, dependently on directions of refractive index distributions, into axial type which has refractive index distribution in the direction along the optical axis, radial type which has refractive index distribution in the direction perpendicular to the optical axis, and so on. Out of these types of GRIN lens elements, the radial type has excellent properties especially for correcting Petzval's sum and chromatic aberration.
Japanese Patent Kokai Publication No. 62-153909 discloses an optical system which uses a GRIN lens element mainly for correcting Petzval's sum. However, this publication makes no sufficient disclosure on the other property to correct chromatic aberration.